The present invention relates to revolving magnetic field type motors.
Japanese Unexamined Patent Publication No. 8-251847 describes a typical revolving field type motor. The motor includes an eight-pole rotor that has a pair of magnet sets each including eight permanent magnets. The motor also includes a stator that has twelve salient poles. A coil is wound around each salient pole. Specifically, the eight permanent magnets of each magnet set are aligned substantially along a circle about the rotational axis of the rotor. The circles defined by the magnet sets are adjacent to each other along the rotor's rotational axis. One magnet set is located offset relative to the other by a predetermined offset angle (7.5 degrees) with respect to the rotational axis of the rotor. This structure reduces cogging torque generated by the motor.
While the rotor of this motor has eight magnetic poles, the stator of the motor has twelve salient poles. The least common multiple between the number of the rotor magnetic poles and the number of the stator salient poles, which is the least common multiple between eight and twelve, is twenty four. If nine salient poles are provided in the motor, instead of twelve, the corresponding least common multiple is seventy two. The least common multiple between the number of the rotor magnetic poles and the number of the stator salient poles corresponds to cogging torque pulsation, or cogging torque frequency. The cogging torque frequency is inversely proportional to cogging torque. That is, if cogging torque frequency increases as the least common multiple increases, cogging torque decreases.
However, if nine salient poles are provided in the motor, the offset angle between the two magnet sets with respect to the rotor's rotational axis must be altered to optimally decrease the cogging torque.
Cogging torque should be reduced in motors other than those having nine salient poles, or motors in which the number of the rotor magnetic poles (P) and the number of the stator salient poles (T) satisfy the following equation: EQU P=8n and T=9n (n is an integer equal to or greater than one).